<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        main {
            width: 800px;
            height: 800px;
            background-color: gray;
            margin: 0 auto;
            border: 1px solid black;
        }

        .hu {
            width: 100px;
            height: 50px;
            background-color: green;
            margin: 50px auto 0 auto;
            position: relative;
            /* overflow: hidden; */
        }

        .son {
            position: absolute;
            bottom: 0;
            left:50%;

            border-radius: 100% 0;

            transform: rotate(45deg);
            background-color: black;
            width: 400px;
            height: 400px;
            /* border: 1px solid red; */
        }
    </style>
</head>

<body>
    <main>
        <div class="hu">
            <div class="son"></div>
        </div>
    </main>
</body>

</html>